An internal combustion engine may experience knock after ignition in one or more engine cylinders has been initiated by a spark and when end gases are ignited by elevated temperatures and pressures in the cylinders. The end gases are not ignited by flame kernels that are produced by the sparks in the cylinders. Engine knock occurs most often at higher engine loads when pressure in the engine's cylinders is high.
Engines may also be operated with a variable number of active or deactivated cylinders, also referred to as variable displacement engines (or VDE), so that engine fuel economy may be increased for a desired engine torque output while optionally maintaining an overall exhaust mixture air-fuel ratio about stoichiometry. In some examples, half of an engine's cylinders may be disabled during selected conditions, where the selected conditions can be defined by parameters such as an engine speed/load window, vehicle speed, etc. In still other examples, cylinders may be individually and selectively deactivated.
If an engine is operated with fewer than all of its cylinders being activated to provide a desired engine torque output, pressures in the active engine cylinders will be higher than pressure in engine cylinders if all engine cylinders had been activated to provide the same desired engine torque. Consequently, the engine may tend to experience knock at lower driver demand torques as compared to if the engine were always operated with all of its cylinders being active.
Engine knock may be controlled on a VDE by activating cylinders and/or retarding spark in active cylinders. However, engine knock is often identified by comparing a background engine noise (e.g., vibration) level against an engine noise level during a crankshaft interval where there is a higher propensity for engine knock. A background engine noise level may decrease when engine cylinders are deactivated and the background engine noise level may increase when deactivated engine cylinders are reactivated. The changing level of background noise may cause the engine knock control system to indicate engine knock when no engine knock is present and it may also cause the engine knock control system to not indicate engine knock when engine knock is present. Therefore, it would be desirable to provide a way of increasing the reliability of the engine knock control system when an engine transitions between VDE modes.
The inventors herein have recognized that engine knock detection may be compromised during VDE mode changes and have developed a method for operating an engine, comprising: operating a variable displacement engine; changing a start of injection (SOI) time from a first SOI time to a second SOI time based on maintaining fuel injector control actions for one or more engine cylinders that are performed during cylinder knock detection windows for engine cylinders when the variable displacement engine is operated with less than all of its cylinders activated.
By changing SOI timing when a new VDE mode is selected, it may be possible to provide the technical result of reducing false indications of engine knock that are due to engine knock background noise levels. In addition, by a cylinder firing pattern from a cylinder firing pattern that has been selected for activation, but has not been activated, to a cylinder firing pattern that maintains engine knock background noise levels of a presently activated cylinder mode, it may be possible to provide the technical result of detecting indications of engine knock right after an engine mode change.
In this way, it may be possible to reduce the possibility of engine degradation. Further, engine fuel economy may be improved when spark is not retarded due to false indications of engine knock. Additionally, other ways of reducing the possibility of false engine knock may be provided by adjusting fuel injection timing and cylinder firing order so that false engine knock indications may be avoided.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.